JP7040390B2 - Backscattered light amplification device, optical pulse test device, backscattered light amplification method, and light pulse test method - Google Patents

Description

The present disclosure relates to a backscattered light amplification device and a backscattered light amplification method for amplifying backscattered light in an optical pulse test for detecting the characteristics of an optical fiber, and an optical pulse test device and an optical pulse test method using the same. ..

As a test technique for an optical fiber, an optical pulse test method (Optical Time Domain Reflectometer, hereinafter OTDR) is well known. In the OTDR, pulsed test light is incident on the optical fiber to be measured (Fiber Under Test, hereinafter FUT), and the backward scattered light or Frenel reflected light of Rayleigh scattered light derived from the test light pulse propagating in the optical fiber. A method and device for acquiring distribution data (OTDR waveform) based on intensity and round trip time. This technique can be used to detect and identify anomalous locations such as broken or increased losses in optical fibers.

In Non-Patent Document 1, a first higher-order mode (LP11 mode) of backscattered light is extracted by using a wavelength region in which a general single-mode fiber (hereinafter SMF) operates in two modes. , A technique for detecting optical fiber bending with higher sensitivity than a general-purpose OTDR (1 μm band mode detection OTDR) is disclosed. Further, in Non-Patent Document 2, the loss caused in the optical fiber is measured by measuring both the basic mode (LP01 mode) and the LP11 mode of the backscattered light in the 1 μm band mode detection OTDR and evaluating the ratio of the loss caused in these. Methods for identifying factors are disclosed.

A. Nakamura, K.K. Okamoto, Y. Koshikiya, T.K. Manabe, M.M. Oguma, T.I. Hashimoto and M. Itoh, “High-sensitivity detection of fiber bends: 1-μm-band mode-detection OTDR”, J. Mol. Lightw. Technol. , Vol. 33, no. 23, pp. 4862-4869, 2015. A. Nakamura, K.K. Okamoto, Y. Koshikiya, T.K. Manabe, M.M. Oguma, T.I. Hashimoto, and M. Iso, "Loss Case Identity By Evaluating Backscattered Modal Loss Ratio Obtained With 1-μm-Band Mode-Destination OTDR", J. Mol. Lightw. Technol. , Vol. 34, no. 15, pp. 3568-3576, 2016. D. M. Spirit and L. C. Blank, "Raman-assisted long-disstance optical time domain reflectometry," Electron. Let. , Vol. 25, pp. 1687-1689, Dec. 1989. Christensen EN, Koefoed JG, Friis SMM, Castanda MAU, Rottwitt K.K. "Experimental characterization of Raman overlaps beween mode-groups," Scientific Reports. 2016; 6: 34693. doi: 10.1038 / rep34693.

In Non-Patent Document 3, as a method for expanding the measurement distance in OTDR measurement, the backward scattered light generated by the probe pulse propagating in the FUT is distributed in the FUT by using the pump light having a higher frequency (short wavelength) by the Raman frequency shift. A method of amplifying the frequency has been proposed.

However, when the FUT is a fumode optical fiber having multiple propagation modes or a general SMF two-mode region, the amplification gain due to induced Raman scattering may differ depending on the propagation modes of the two interacting lights. It is known (Non-Patent Document 4). Therefore, in OTDR measurement of a fumode optical fiber or a general SMF in a two-mode region, there is a problem that a method for amplifying a desired propagation mode of backscattered light with a desired gain via induced Raman scattering is unknown. be.

Therefore, in order to solve the above-mentioned problems of the prior art, the present invention amplifies a desired propagation mode of backward Rayleigh scattered light with a desired gain by induced Raman scattering in a measured optical fiber having a plurality of propagation modes. It is an object of the present invention to provide a scattered light amplification device, an optical pulse test device, a backward scattered light amplification method, and an optical pulse test method.

In order to achieve the above object, the backscattered light amplification device according to the present invention propagates when a probe pulse is input to the optical fiber to be measured in an arbitrary propagation mode and then a pump pulse is incident in a plurality of propagation modes. It was decided to individually control the power, incident timing, and pulse width of the pump pulse for each mode.

Specifically, the backscattered light amplification device according to the present invention is
A probe pulse incident means for incident the probe pulse on one end of the optical fiber to be measured in a desired propagation mode,
After the probe pulse incident means incidents the probe pulse on the optical fiber to be measured, a pump pulse that generates a Raman gain spectrum in an optical frequency range including the optical frequency of the probe pulse is generated in a plurality of propagation modes. A pump pulse incident means incident on the one end of the fiber,
Of the plurality of propagation mode backward scattered light generated by the probe pulse propagating in the measured optical fiber, the desired propagation mode backward scattered light generated farther from a desired point of the measured optical fiber is desired. The power ratio of the pump pulse between the propagation modes, the length of the pump pulse in each propagation mode, and the probe pulse incident on the optical fiber to be measured and the pump pulse in each propagation mode so as to give Raman amplification gain. Control means for setting the relative time difference with
To prepare for.

Further, the backscattered light amplification method according to the present invention is
The procedure for injecting the probe pulse into one end of the optical fiber to be measured in the desired propagation mode, and the procedure for injecting the probe pulse.
After the probe pulse is incident on the optical fiber to be measured in the probe pulse incident procedure, a pump pulse that generates a Raman gain spectrum in an optical frequency range including the optical frequency of the probe pulse is generated in a plurality of propagation modes. The procedure for injecting a pump pulse incident on the one end of the fiber,
In the pump pulse incident procedure, among the backward scattered light of the plurality of propagation modes generated by the probe pulse propagating in the optical fiber to be measured, the desired propagation mode generated farther from the desired point of the optical fiber to be measured. With the power ratio of the pump pulse between propagation modes, the length of the pump pulse in each propagation mode, and the probe pulse incident on the optical fiber to be measured so as to give the desired Raman amplification gain to the backscattered light of. A control procedure for setting the relative time difference between the pump pulse and the pump pulse in each propagation mode, and
I do.

By individually controlling the power, incident timing, and pulse width of the pump pulse for each propagation mode, any Raman amplification gain can be obtained at any point (subject) for the desired propagation mode of backscattered light generated by the probe pulse. It can be given from a point in the longitudinal direction of the measurement optical fiber).

Therefore, the present invention is a backward scattered light amplification device that amplifies a desired propagation mode of rear Rayleigh scattered light with a desired gain by induced Raman scattering in an optical fiber to be measured in which a plurality of propagation modes exist, and a backward scattered light amplification device. A method can be provided.

The backscattered light amplification device according to the present invention further includes a mode demultiplexing means for separating the backscattered light returned to the one end of the measured optical fiber for each propagation mode.
Further, the backscattered light amplification method according to the present invention further performs a mode demultiplexing procedure for separating the backscattered light returned to the one end of the measured optical fiber for each propagation mode.

The optical pulse test device according to the present invention includes the backscattered light amplification device and the backscattered light amplification device.
An arithmetic processing device that acquires the light intensity distribution in the length direction of the measured optical fiber from the backscattered light returning to the one end of the measured optical fiber for each propagation mode.
It is an optical pulse test device equipped with
The arithmetic processing unit is
By operating the backscattered light amplification device, the first light intensity distribution when the probe pulse and the pump pulse are incident on the optical fiber to be measured is acquired.
By operating the pump pulse incident means and the mode demultiplexing means of the backscattered light amplification device, the second light intensity distribution when only the pump pulse is incident on the optical fiber to be measured is acquired.
For each propagation mode, the second light intensity distribution is subtracted from the first light intensity distribution to obtain the third light intensity distribution that would occur when only the probe pulse is incident on the optical fiber to be measured. It is a feature.

The optical pulse test method according to the present invention includes the backscattered light amplification method and the method.
An arithmetic processing method for acquiring the light intensity distribution in the length direction of the optical fiber to be measured from the backscattered light returning to the one end of the optical fiber to be measured for each propagation mode.
Is an optical pulse test method
By the backscattered light amplification method and the arithmetic processing method, the first light intensity distribution when the probe pulse and the pump pulse are incident on the optical fiber to be measured is acquired.
The second light intensity distribution when only the pump pulse is incident on the optical fiber to be measured is acquired by the pump pulse incident procedure, the mode demultiplexing procedure, and the arithmetic processing method of the backscattered light amplification method.
For each propagation mode, the second light intensity distribution is subtracted from the first light intensity distribution to obtain the third light intensity distribution that would occur when only the probe pulse is incident on the optical fiber to be measured. It is a feature.

This backscattered light amplification device and its method can exclude the backscattered light component of the pump pulse, which is a noise component, and observe the backscattered light in a desired propagation mode.

The above inventions can be combined as much as possible.

The present invention is a backward scattering light amplification device, an optical pulse test device, and a backward scattering device that amplify a desired propagation mode of rear Rayleigh scattered light with a desired gain by induced Raman scattering in a measured optical fiber having a plurality of propagation modes. An optical amplification method and an optical pulse test method can be provided.

It is a figure explaining the structure of the optical pulse test apparatus which concerns on this invention. It is a figure explaining the mode combiner / demultiplexer provided in the optical pulse test apparatus which concerns on this invention. It is a figure explaining the induction Raman amplification by the backscattered light amplification apparatus which concerns on this invention. It is a figure explaining the induction Raman amplification by the backscattered light amplification apparatus which concerns on this invention. It is a figure explaining the optical frequency arrangement of each light by the backscattered light amplification apparatus which concerns on this invention. It is a figure explaining the optical pulse test method which concerns on this invention.

An embodiment of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and the drawings, the components having the same reference numerals indicate the same components.

FIG. 1 is a diagram illustrating an optical pulse test apparatus of the present embodiment. In this embodiment, it is assumed that the optical fiber to be measured propagates only in two propagation modes, the basic mode and the first higher-order mode, and the propagating light is in a unipolar state. This optical pulse test device acquires the light intensity distribution in the length direction of the measured optical fiber FUT from the backscattered light amplification device 10 and the backscattered light returning to one end of the measured optical fiber FUT for each propagation mode. The device 20 is provided.

The backscattered light amplification device 10 is
The probe pulse incident means 11 for incident the probe pulse P0 on one end of the optical fiber FUT to be measured in a desired propagation mode, and the probe pulse incident means 11.
After the probe pulse incident means 11 injects the probe pulse P0 into the optical fiber FUT to be measured, a plurality of propagation modes of pump pulses (P1, P2) that generate a Raman gain spectrum in the optical frequency range including the optical frequency of the probe pulse P0. With the pump pulse incident means 12 incident on the one end of the optical fiber FUT to be measured,
Of the multiple propagation mode rearscattered lights generated by the probe pulse P0 propagating in the measured optical fiber FUT, the desired propagating mode backward scattered light generated farther than the desired point of the measured optical fiber FUT is desired. The power ratio of the pump pulse between the propagation modes, the length of the pump pulse in each propagation mode, and the relative of the probe pulse P0 incident on the optical fiber FUT to be measured and the pump pulse in each propagation mode so as to give Raman amplification gain. The control means 13 for setting the target time difference (ΔT1, ΔT2) and
To prepare for.

The backscattered light amplification device 10 further includes a mode demultiplexing means (mode combined demultiplexer 1-13) for separating the backscattered light returned to the one end of the optical fiber FUT to be measured for each propagation mode.

In FIG. 1, 1-1 is a first light source that emits light having an optical frequency ν1, and 1-2 is a first optical pulser that pulses light emitted from a first light source to generate a probe pulse. 1-3 is a second light source that emits light with an optical frequency ν2, and 1-4 is a variable optical brancher that branches the light emitted from the second light source at a desired ratio, 1-5 and 1-6. Is a second and third optical pulser that pulses the light branched by the variable optical brancher to generate first and second pump pulses, and 1-7, 1-8 and 1-9 are the first. An electric pulse generator that generates an electric pulse to modulate the light input to each of the second and third optical pulsers, 1-10 is an optical combination that combines the probe pulse and the first pump pulse. The wave device, 1-11 is the first optical circulator that separates the basic mode component of the backward scattered light, 1-12 is the second optical circulator that separates the first higher-order mode component of the backward scattered light, and 1-13 is the second optical circulator. The multiplexed probe pulse, the first pump pulse, and the second pump pulse are incident on the measured optical fiber in the basic mode and the first higher-order mode, respectively, and the back-scattered light from the measured optical fiber is basically used. The mode combiner / demultiplexer, 1-14 and 1-15, which separates the mode component and the first higher-order mode component, is the light that removes the backward scattered light generated by the first and second pump pulses from the backward scattered light. Band path filters, 1-16 and 1-17 represent optical detectors, 1-18 and 1-19 represent A / D converters, and 1-20 represent arithmetic processors.

The probe pulse incident means 11 includes a first light source 1-1, a first optical pulser 1-2, an electric pulse generator 1-7, an optical combiner 1-10, and a first optical circulator 1-11. And mode combiner / demultiplexer 1-13.
The pump pulse incident means 12 includes a second light source 1-3, a variable optical brancher 1-4, a second optical pulser 1-5, a third optical pulser 1-6, and an electric pulse generator 1. -8, an electric pulse generator 1-9, an optical combiner 1-10, a first optical circulator 1-11, a second optical circulator 1-12, and a mode combiner / demultiplexer 1-13.
The control means 13 controls the variable optical turnout 1-4, the electric pulse generator 1-7, the electric pulse generator 1-8, and the electric pulse generator 1-9.
The arithmetic processing unit 20 includes an optical bandpass filter (1-14, 1-15), a photodetector (1-16, 1-17), an A / D converter (1-18, 1-19), and an arithmetic. It is a processor 1-20.

Since the gain bandwidth due to induced Raman scattering is relatively wide, in this optical pulse test device, the rear Rayleigh scattered light of the probe pulse receives sufficient amplification gain even if the spectral line width is wide, so the probe pulse light source is used. A general-purpose DFB laser or the like can be used as the first light source 1-1. In this case, heterodyne detection cannot be used, so direct detection will be used.

Theoretically, in the backscattered light amplification device 10, the gain of induced Raman scattering generated by the first and second pump pulses can be controlled and amplified with respect to the desired propagation mode of the backscattered light generated by the probe pulse. explain.

The light having an optical frequency ν1 emitted from the first light source 1-1 is pulsed by an optical pulser 1-2 based on the electric signal generated by the electric pulse generator 1-7, and is a probe pulse. Is generated. On the other hand, the light having the optical frequency ν2 emitted from the second light source 1-3 is branched at a desired ratio by the variable optical branching device 1-4, and is generated by the electric pulse generators 1-8 and 1-9, respectively. Based on the generated electric signal, it is pulsed by optical pulsers 1-5 and 1-6 to generate first and second pump pulses. The frequency difference Δν = ν2-ν1 between the optical frequency ν1 of the probe pulse and the optical frequency ν2 of the first and second pump pulses is set so as to match the Raman gain band of the optical fiber to be measured.

The first pump pulse is multiplexed by the probe pulse and the optical duplexer 1-10, passes through the first optical circulator 1-11, and is input to the mode duplexer 1-13. The second pump pulse passes through the second optical circulator 1-12 and is input to the mode duplexer 1-13.

As shown in FIG. 2, for example, the mode duplexer 1-13 outputs the LP01 mode input to the port 1 to the port 3 as the LP01 mode, and changes the LP01 mode input to the port 2 to the LP11 mode. Convert and output from port 3. Further, the LP01 mode and the LP11 mode input to the port 3 are output to the port 1 in the LP01 mode as the LP01 mode, and are converted into the LP01 mode in the LP11 mode and output to the port 2. When such a mode duplexer 1-13 is used for the backward scattered light amplification device 10, the first optical circulator 1-11 is connected to port 1 and the second optical circulator 1-12 is connected to port 2. By connecting, as described above, the multiplexed probe pulse and the first pump pulse can be incident on the optical fiber FUT to be measured in the LP01 mode and the second pump pulse in the LP11 mode. Further, by connecting the second optical circulator 1-12 to the port 1 and connecting the first optical circulator 1-11 to the port 2, contrary to the above, the multiplexed probe pulse and the first optical circulator are connected. The pump pulse can be incident on the optical fiber FUT to be measured in the LP11 mode and the second pump pulse can be incident on the optical fiber FUT in the LP01 mode.

The multiplexed probe pulse, the first pump pulse, and the second pump pulse are input from one end of the optical fiber FUT to be measured. The probe pulse is incident in time first, followed by a first pump pulse with a time delay ΔT1 and a second pump pulse with a time delay ΔT2. The probe pulse, the pulse width of the first and second pump pulses, the relative time delay between the probe pulse and the first and second pump pulses ΔT1 and ΔT2 are the electric pulse generators 1-7, 1-8 and 1-9. It can be adjusted by.

As shown in FIG. 3, when the probe pulse and the first and second pump pulses are incident on the optical fiber FUT to be measured, backscattered light is first generated by the previously incident probe pulse. For example, when the wavelength of the probe pulse is in a two-mode region (basic mode and first higher-order mode can propagate) below the cutoff wavelength of a general single-mode optical fiber, the backscattered light is in the basic mode and the first mode. It is coupled to a higher-order mode and propagated to the input end side of the optical fiber FUT to be measured. Since Rayleigh scattering is elastic scattering, the optical frequency does not change due to the scattering process, and the generated backscattered light and the probe pulse have the same optical frequency ν1.

Subsequently, the first and second pump pulses propagate in the optical fiber FUT to be measured so as to follow the probe pulse. When the backscattered light generated by the probe pulse meets the pump pulse and the backscattered light is within the Raman amplification gain band generated by the pump pulse, induced Raman amplification occurs from the pump pulse to the backscattered light and the backscattered light is Amplified.

As shown in FIG. 4, when the optical fiber FUT to be measured is an optical fiber having multiple propagation modes, or each pulse is in a wavelength region where multiple propagation modes propagate, the basic mode of backward scattered light is LP01 mode. It receives a predetermined Raman amplification gain from both the input pump pulse (first pump pulse) and the pump pulse input in LP11 mode (second pump pulse). In addition, the LP11 mode of backscattered light also receives a predetermined Raman amplification gain from both the pump pulse (first pump pulse) input in the LP01 mode and the pump pulse (second pump pulse) input in the LP11 mode. It will be.

After separating the basic mode and the first higher-order mode of the backscattered light returning to the incident end of the optical fiber FUT to be measured by the mode duplexer 1-13, it is passed through the circulators 1-11 and 1-12. After the backscattered light is extracted and the backscattered light generated in the first and second pump pulses is removed by the optical band pulse filters 1-14 and 1-15, the light detectors 1-16 and 1-17 are used for electricity. It is converted into a signal, digitized by A / D converters 1-18 and 1-19, and analyzed by an arithmetic processor 1-20.

および

で表される。ここで、Ｐ_０（０）はプローブパルスの入射端での光パワー、α_０１は被測定光ファイバＦＵＴのＬＰ０１モードの損失係数、Ｒ_{０１－０１}およびＲ_{０１－１１}はプローブパルスのＬＰ０１モードが後方散乱光のＬＰ０１モードおよびＬＰ１１モードに結合する割合である。 When the input end (one end) of the optical fiber FUT to be measured is z = 0, the powers P 01 (z) and P 11 (z) of the backscattered light generated by the probe pulse at the point z are the powers P ₀₁ (z) and P ₁₁ (z) of the LP 11 mode. ,

and

It is represented by. Here, P ₀ (0) is the optical power at the incident end of the probe pulse, α ₀₁ is the loss coefficient of the LP01 mode of the optical fiber FUT to be measured, and R _01-01 and R _01-11 are the LP 01 mode of the probe pulse. The ratio of the backscattered light to the LP01 mode and the LP11 mode.

ここで、γ０１－０１およびγ１１－０１は後方散乱光のＬＰ０１モードが第１（ＬＰ０１モード）および第２（ＬＰ１１モード）のポンプパルスから誘導ラマン散乱により利得をうけるモード依存利得効率を表す。ηは可変光分岐器１－４の分岐比、すなわち第１および第２のポンプパルスのパワーの比率である。

は地点ｚ－Δｚ１における第１（ＬＰ０１モード）のポンプパルスの光パワーであり、

は地点ｚ－Δｚ２における第２（ＬＰ１１モード）のポンプパルスの光パワーである。
ΔＬ１およびΔＬ２はプローブパルスの後方散乱光が第１および第２のポンプパルスにより誘導ラマン増幅される相互作用長であり、ポンプパルスのパルス幅によって調整可能である。 The LP01 mode of the backscattered light returning to the input end receives the following Raman amplification G01 from the first (LP01 mode) and the second (LP11 mode) pump pulses.

Here, γ01-01 and γ11-01 represent mode-dependent gain efficiencies in which the LP01 mode of backscattered light receives a gain from the first (LP01 mode) and the second (LP11 mode) pump pulses by induced Raman scattering. η is the branch ratio of the variable optical turnouts 1-4, that is, the ratio of the powers of the first and second pump pulses.

Is the optical power of the first (LP01 mode) pump pulse at point z-Δz1.

Is the optical power of the second (LP11 mode) pump pulse at point z−Δz2.
ΔL1 and ΔL2 are interaction lengths in which the backscattered light of the probe pulse is induced Raman amplified by the first and second pump pulses, and can be adjusted by the pulse width of the pump pulse.

ここで、γ０１－１１およびγ１１－１１は後方散乱光のＬＰ１１モードが第１（ＬＰ０１モード）および第２（ＬＰ１１モード）のポンプパルスから誘導ラマン散乱により利得をうけるモード依存利得効率を表す。 Further, the LP11 mode of the backscattered light receives the following Raman amplification G11 from the first (LP01 mode) and the second (LP11 mode) pump pulses.

Here, γ01-11 and γ11-11 represent mode-dependent gain efficiencies in which the LP11 mode of backscattered light receives gain from the first (LP01 mode) and second (LP11 mode) pump pulses by induced Raman scattering.

これは被測定光ファイバＦＵＴの屈折率プロファイルによって一意に決定される。 The mode-dependent gain efficiency, that is, the Raman amplification efficiency between different modes is an overlap integral of the cross-sectional strength distributions I of the modes n and m, and is expressed by the following equation.

This is uniquely determined by the refractive index profile of the optical fiber FUT to be measured.

および

ここで、ｃは被測定光ファイバＦＵＴ中の光速度を表す。 The distances (1/2 of the pulse interval) Δz1 and Δz2 from the position where the backscattered light of the probe pulse is generated to the respective positions where the first and second pump pulses are met are calculated by the following equations using the time delays ΔT1 and ΔT2. It is represented by.

and

Here, c represents the speed of light in the optical fiber FUT to be measured.

By controlling the time intervals ΔT1 and ΔT2 between the probe pulse and the pump pulse, it is possible to amplify the probe light from an arbitrary point. The speed of light in the optical fiber FUT to be measured changes slightly depending on the propagation mode, but normally the pulse width (interaction length with backward scattered light) of the pump pulse is 10 km or more, whereas each mode Since the change in Δz1 and Δz2 caused by the difference in the speed of light is less than 10 m, the difference in the speed of light for each mode can be ignored.

一方、後方散乱光のＬＰ１１モードのパワーＰ_１１（ｔ）は、以下のように表される。

ここで、α_１１は被測定光ファイバＦＵＴのＬＰ１１モードの損失係数である。また、

である。 The power P ₀₁ (t) in the LP01 mode of the backscattered light generated by the probe pulse observed at the receiving unit (z = 0) of the present device at time t is expressed as follows.

_On the other hand, the power P11 (t) of the backscattered light in the LP11 mode is expressed as follows.

Here, α ₁₁ is the loss coefficient of the LP11 mode of the optical fiber FUT to be measured. again,

Is.

As shown in equations (8) and (9), the power ratio η of the first and second pump pulses, the interaction length between the probe pulse and the first and second pump pulses ΔL1, ΔL2, the probe pulse and the second By controlling the time difference ΔT1 and ΔT2 relative to the first and second pump pulses, any Raman amplification gain can be given from any point to any mode of the backscattered light of the probe pulse. ..

FIG. 5 schematically shows the frequency arrangement relationship between the probe pulse, the pump pulse, the backscattered light generated by each pulse, and the Raman gain spectrum generated by the pump pulse.

The backward Rayleigh scattered light generated by the probe pulse has the same optical frequency ν1 as the probe pulse. For a pump pulse having an optical frequency ν2, the Raman gain spectrum is centered on ν2-νr downshifted by the Raman frequency shift νr of the optical fiber FUT to be measured. When the frequency ν1 of the probe pulse is within the Raman gain spectrum, the backscattered light of the probe pulse is Raman amplified in the optical fiber FUT to be measured.

FIG. 6 is a flowchart illustrating a waveform analysis procedure in the arithmetic processor 1-20.

First, in step S01, the LP01 mode waveform b1 and the LP11 mode waveform b2 of the backscattered light are acquired with the probe pulse and the first and second pump pulses input. Let the waveforms b1 and b2 observed at this time be _FG ⁰¹ (z) and _FG ¹¹ (z), respectively.
Next, in step S02, the LP01 mode waveform b3 and the LP11 mode waveform b4 of the rear Raman scattered light having no inductive component generated only by the pump pulse while only the first and second pump pulses are input are acquired. The waveforms observed at this time are _FP ⁰¹ (z) and _FP ¹¹ (z), respectively.
Finally, in step S03, by taking the differences b1-b3 and b2-b4 of these waveforms, the _LP01 mode waveform FR ⁰¹ (z) and the _LP11 mode waveform FR of the rear Rayleigh scattered light of the amplified probe pulse are taken. ¹¹ (z) can be acquired and analyzed.
The waveforms _FR ⁰¹ (z) and the waveform _FR ¹¹ (z) are obtained by using the equations (8) and (9) as functions of the length direction z of the optical fiber FUT to be measured, respectively.

[Additional Notes]
The following describes the optical pulse test apparatus according to the present invention.
(Task)
To enable the backscattered light of multiple propagation modes propagating in an optical fiber to be individually amplified by induced Raman scattering.

(means)
In order to solve the above problems, this optical pulse test device
A first light source that outputs probe light having a wavelength that can be propagated in the basic mode and the first higher-order mode of the optical fiber FUT to be measured, and
A pulser that pulses the probe light and generates a probe pulse,
A second light source that outputs pump light having a wavelength shifted to a shorter wavelength by the Raman frequency shift than the probe light, and a second light source.
A variable optical turnout that branches the pump light at a desired branch ratio,
Controls the second and third pulsers and the first, second and third pulsers that pulse the pump light branched by the variable optical turnout to generate the first and second pump pulses. With a signal generator that produces an electric pulse train for
An optical combiner that combines the probe pulse and the first pump pulse,
A first optical circulator for separating the basic mode component of the backscattered light from the optical fiber FUT to be measured, and
A second optical circulator for separating the first higher-order mode of the backscattered light from the optical fiber FUT to be measured, and
The combined probe pulse and the first pump pulse are measured in either the basic mode or the first higher-order mode, and the second pump pulse is measured in either the basic mode or the first higher-order mode, respectively. A mode duplexer that is incident on the optical fiber FUT and separates the backscattered light from the optical fiber FUT to be measured into the basic mode and the first higher-order mode.
A first optical bandpass filter that removes the backscattered light of the pump pulse from the basic mode of the backscattered light separated by the first optical circulator.
A second optical bandpass filter that removes the backscattered light of the pump pulse from the first higher-order mode of the backscattered light separated by the second optical circulator.
A first and second photodetector that photoelectrically converts backscattered light that has passed through the first and second optical bandpass filters, respectively.
A first and second A / D converter that converts the photocurrent output from the first and second photodetectors into a voltage, respectively.
An arithmetic processing unit that acquires the intensity distribution of the basic mode component of the backscattered light with respect to the distance of the optical fiber FUT to be measured and the intensity distribution of the first higher-order mode component of the return light with respect to the distance of the assumed optical fiber. ,
To prepare for.

The pump light has a wavelength shifted to a shorter wavelength by the Raman frequency shift than the probe pulse, and is mode-selective by the variable optical branching device, the second and third pulsers, and the mode combiner / demultiplexer. The pump pulse excited by is controlled to an arbitrary optical power ratio, pulse width, and relative time delay with the probe pulse, and the basic mode of the rear Rayleigh scattered light of the probe pulse previously incident on the optical fiber FUT to be measured and the second 1 It is characterized in that a higher-order mode is amplified from a desired point with a desired gain.

In the first measurement, the arithmetic processor acquires a backscattered light waveform with the probe pulse and the first and second pump pulses incident on the optical fiber FUT to be measured.
In the second measurement, the backscattered light waveforms of the first and second pump pulses are acquired in a state where the probe pulse is not incident on the optical fiber FUT to be measured.
It is characterized in that the rear Rayleigh scattered light waveform of the probe pulse is calculated from the difference between the waveform acquired in the first measurement and the waveform acquired in the second measurement.

(The invention's effect)
According to the optical pulse test apparatus and the optical pulse test method according to the present invention, the conditions (input mode) of the pump light to be input in the optical fiber having a plurality of propagation modes or in the wavelength region where each pulse propagates in a plurality of propagation modes. , Input power, input timing, pulse width), the induced Raman scattering can be used to amplify the desired propagation mode of the rear Rayleigh scattered light from the desired point with the desired gain.
In the present embodiment, it has been described that the optical fiber to be measured propagates only in two propagation modes, the basic mode and the first higher-order mode, and the propagating light is in a unipolar state. Even if the fiber can propagate in three or more propagation modes and the propagating light is in a single polarization state, the desired propagation mode of the rear Rayleigh scattered light is desired by controlling the conditions of the input pump light in the same manner. It can be amplified from a desired point with the gain of.

1-1: 1st light source 1-2: 1st pulser 1-3: 2nd light source 1-4: Variable optical brancher 1-5, 1-6: 2nd, 3rd pulse Instrument 1-7 to 1-9: Electric pulse generator 1-10: Optical duplexer 1-11, 1-12: Optical circulator 1-13: Mode duplexer 1-14, 1-15: Optical band pal Filters 1-16, 1-17: Optical detectors 1-18, 1-19: A / D converter 1-20: Arithmetic processor No. 10: Backscattered light amplification device 11: Probe pulse incident means 12: Pump pulse Incident means 13: Control means 20: Arithmetic processing device

Claims (6)

A probe pulse incident means for incident the probe pulse on one end of the optical fiber to be measured in a desired propagation mode,
After the probe pulse incident means incidents the probe pulse on the optical fiber to be measured, a pump pulse that generates a Raman gain spectrum in an optical frequency range including the optical frequency of the probe pulse is generated in a plurality of propagation modes. A pump pulse incident means incident on the one end of the fiber,
Of the plurality of propagation mode backward scattered light generated by the probe pulse propagating in the measured optical fiber, the desired propagation mode backward scattered light generated farther from a desired point of the measured optical fiber is desired. The power ratio of the pump pulse between the propagation modes, the length of the pump pulse in each propagation mode, and the probe pulse incident on the optical fiber to be measured and the pump pulse in each propagation mode so as to give Raman amplification gain. Control means for setting the relative time difference with
A backscattered light amplifier equipped with. The backscattered light amplification device according to claim 1, further comprising a mode demultiplexing means for separating the backscattered light returned to the one end of the optical fiber to be measured for each propagation mode. The backscattered light amplification device according to claim 2,
An arithmetic processing device that acquires the light intensity distribution in the length direction of the measured optical fiber from the backscattered light returning to the one end of the measured optical fiber for each propagation mode.
It is an optical pulse test device equipped with
The arithmetic processing unit is
By operating the backscattered light amplification device, the first light intensity distribution when the probe pulse and the pump pulse are incident on the optical fiber to be measured is acquired.
By operating the pump pulse incident means and the mode demultiplexing means of the backscattered light amplification device, the second light intensity distribution when only the pump pulse is incident on the optical fiber to be measured is acquired.
For each propagation mode, the second light intensity distribution is subtracted from the first light intensity distribution to obtain the third light intensity distribution that would occur when only the probe pulse is incident on the optical fiber to be measured. A featured optical pulse test device. The procedure for injecting the probe pulse into one end of the optical fiber to be measured in the desired propagation mode, and the procedure for injecting the probe pulse.
After the probe pulse is incident on the optical fiber to be measured in the probe pulse incident procedure, a pump pulse that generates a Raman gain spectrum in an optical frequency range including the optical frequency of the probe pulse is generated in a plurality of propagation modes. The procedure for injecting a pump pulse incident on the one end of the fiber,
In the pump pulse incident procedure, among the backward scattered light of the plurality of propagation modes generated by the probe pulse propagating in the optical fiber to be measured, the desired propagation mode generated farther from the desired point of the optical fiber to be measured. With the power ratio of the pump pulse between propagation modes, the length of the pump pulse in each propagation mode, and the probe pulse incident on the optical fiber to be measured so as to give the desired Raman amplification gain to the backscattered light of. A control procedure for setting the relative time difference between the pump pulse and the pump pulse in each propagation mode, and
Backscattered light amplification method. The backscattered light amplification method according to claim 4, further comprising a mode demultiplexing procedure for separating the backscattered light returned to the one end of the optical fiber to be measured for each propagation mode. The backscattered light amplification method according to claim 5,
An arithmetic processing method for acquiring the light intensity distribution in the length direction of the optical fiber to be measured from the backscattered light returning to the one end of the optical fiber to be measured for each propagation mode.
Is an optical pulse test method
By the backscattered light amplification method and the arithmetic processing method, the first light intensity distribution when the probe pulse and the pump pulse are incident on the optical fiber to be measured is acquired.
The second light intensity distribution when only the pump pulse is incident on the optical fiber to be measured is acquired by the pump pulse incident procedure, the mode demultiplexing procedure, and the arithmetic processing method of the backscattered light amplification method.
For each propagation mode, the second light intensity distribution is subtracted from the first light intensity distribution to obtain the third light intensity distribution that would occur when only the probe pulse is incident on the optical fiber to be measured. A characteristic optical pulse test method.

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